We have continued studies of chromatin structure, with respect both to the problem of packaging DNA complactly in eukaryotic nuclei, and the question of structural changes associated with gene expression. We have shown that 30 nm chromatin fibers from a variety of sources, and with different nucleosome subunit repeats, nonetheless have a similar packing of nucleosomes within the fiber. This structure is thus a nearly invariant feature of the nucleosomes within the fiber. We have shown that structures of this kind are present even in the neighborhood of genes that are being expressed. We have also shown that histone hyperacetylation, thought to be associated with gene activation, has almost no effect on the stability of the 30 nm fiber. In other studies, we have examined the structure of DNA in the promoter region of the chicken adult beta globin gene, which we have shown has a special chromatin structure when the gene is expressed. We find an unusual region consisting of 16 deoxyguanosines which is sensitive to attack by a single-strand-specific nuclease, and which may be important in determining the properties of chromatin in this region.